Suggested design of gold-nanoobjects-based terahertz radiation source for biomedical research

Abstract

Gold nanoparticles (GNPs) may serve as devices to emit electromagnetic radiation in the terahertz (THz) range, whereby the energy is delivered by radio frequency or microwave photons which will not by themselves induce transitions between sparse confinement-shaped electron levels of a GNP, but may borrow the energy from longitudinal acoustic (LA) phonons to overcome the confinement gap. Upon excitation, the Fermi electron cannot relax otherwise than via emitting a THz photon, the other relaxation channels being blocked by force of shape and size considerations. Within this general scope that has already been outlined earlier, the present work specifically discusses two-phonon processes, namely (i) a combined absorption-emission of two phonons from the top of the LA branch, and (ii) an absorption of two such phonons with nearly identical wavevectors. The case (i) may serve as a source of soft THz radiation (at ≃0.54 THz), the case (ii) the hard THz radiation at 8.7 THz. Numerical estimates are done for crystalline particles in the shape of rhombicuboctahedra, of 5-7 nm size. A technical realisation of this idea is briefly discussed, assuming the deposition of GNPs onto/within the substrate of Teflon, the material sustaining high temperatures and transparent in the THz range.

Prospects for Terahertz Imaging the Human Skin Cancer with the Help of Gold-Nanoparticles-Based Terahertz-to-Infrared Converter

“…Further on, in Ref. 24 we have shown that gold nanorhombicuboctahedra could be used as emitters of radiation at "soft" (two-phonon difference frequency) 0.54 THz and at "hard" (summary frequency) 8.7 THz. The 0.5 THz emission is important for the THz imaging of human skin cancer, due to maximal contrast it yields between the cancer and the normal tissue, in the context of findings of Wallace and Pickwell 19,20 .…”

Prospects of designing gold-nanoparticles-based soft terahertz radiation sources and terahertz-to-infrared converters for concealed object detection technology

“…2 in Ref. 25 parametrised the experimental Γ − X dispersion relation for bulk gold and elaborated on the matching conditions between the energy of absorbed/released phonons, ω(q 1 )− ω(q 2 ), and the quantisation step on the electron excitation.…”

“…Such an analysis has been done in Ref. 25, albeit with compact GNPs (not elongated GNBs) in mind. The discussion around Fig.…”

A device to inspect a skin cancer tumour in the terahertz range, transferring the image into the infrared

“…In Ref. [10] it was shown that gold nanorhombicuboctahedra could be used as emitters of radiation at 0.54 and at 8.7 THz, important for the THz imaging of human skin cancer, in the context of findings of Ref. [11,12] and possible impact on the improvement of spatial resolution.…”